Switched reluctance motors are simple in construction and do not require permanent magnets or rotor coils/slip rings to function. They are therefore cheap and attractive as a potential for hybrid and electric vehicle usage.
Switched reluctance motor function is well understood and typically operates under a 3 phase (A,B,C phase) arrangement where motion is produced as a result of the variable reluctance in the air gap between the rotor and the stator. When a stator winding is energized, producing a single magnetic field, reluctance induced torque is produced by the tendency of the rotor to move to its minimum reluctance position.
Magnetic pole switching is controlled via an inverter to regulate the speed of the rotating magnetic field around the motor and thus the actual speed/torque of the motor output.
Switched reluctance motors have long suffered from NVH (Noise, Vibration and Harshness) problems during use due to high loads between the rotor and stator which in turn cause high mechanical stresses in the casing. This problem can be exacerbated by the high phase switching frequencies and the motor speed itself. Case noise can be generated from radial forces building up between opposing poles—when the inverter switches between one phase and the next, the force is quickly released radially which in turn manifests as a vibration in the stator case. This effect produces direct case generated noise and is the focus of prior art document JP4797227B2.
As well as direct motor casing derived noises there are other possible sources of motor related noise such as vehicle and referred driveline vibrations caused by variation in angular motor torque output. As a motor switches between phases as it rotates there is a fluctuation in motor output torque which is known as torque ripple.
During rotor rotation and when a rotor tooth is approaching the next mating stator tooth/pole the magnetic phase is switched on for that pole pair and the rotor and stator become attracted to each other as the stator is acting like a magnet. The tangential forces exerted across the airgap between the rotor and stator act on the rotor to turn the shaft. The angular torque applied to the rotor shaft varies due to switching current/timing, magnetic field strength and tooth geometry. The variation in torque applied while the motor runs is called torque ripple and this invention seeks to reduce torque ripple and its associated noise problems—for example it may be that the torque ripple of a standard reluctance motor may generate torque output oscillations which are able to excite connected rotating shafts or gears in the driveline, or indeed casings around the rotating parts, which are affected by the referred oscillations. It is also possible that motor torque output oscillations can induce vibrations into the vehicle body shell via driveline mounted components on the body shell. Separately mounted subsystem components on the body shell can be excited also such as pumps or heat shields for example.
It is this problem of motor torque ripple which this invention seeks to address.
Both torsional and radial forces are present between the rotor and stator as the motor rotates. The forces and their direction are determined by the flow direction and density of the flux lines created between the rotor tooth and stator/pole as one sweeps past the other. These lines of flux or force and their magnitude change constantly but can be modelled using available software.
By influencing the direction and density of the flux lines in a timed sense, the torque output characteristics of the motor can be changed significantly so as to reduce torque ripple significantly but without excessive loss of motor torque capability. This invention seeks to outline how these lines of force can be modified in a new way.
Motor stators and rotors are typically manufactured by building up layers of pressed ferrous material forming laminations; these laminations are then bonded or held together using conventional means to form longer stacks. These stacks are typically solid with no voids or air gaps so as to allow the magnetic flux or flux lines to flow unhindered from rotor to stator during motor operation. The laminations are generally made of special electrical steel which may have added silicon and be rolled in such a way as to optimise the flux density in the rolling direction. Using electrical steel can increase flux density by up to 30%. The steel magnetic properties are further influenced by the metallurgical crystal size which is controlled during heat treatment.
Each lamination can also be coated to increase electrical resistance between laminations and thus to reduce eddy currents and losses.
It is known to deliberately include continuous voids or flux barriers in rotor and stator constructions to influence magnetic flux paths during operation of the motor. This is shown in patent JP4797227B2 where the focus is to reduce the deformation of the stator case due to radial forces generated when the rotor is spinning.
A flux barrier may be formed using a void which could be filled with air or another non magnetic material.
It is the aim of the present invention to provide improvements to switched reluctance motor design to reduce the amplitude and duration of torque ripple, resulting in a reduction in motor output torsional oscillations and therefore improvements in referred noise and vibration problems which can be transmitted to other parts of the vehicle.